Conclusion: Sensitivity of DNA to digestion by DNase I is correlated with gene expression, suggesting that chromatin structure changes during transcription. _____

11.9 DNase I sensitivity is correlated with the transcription of globin genes in erythroblasts of chick embryos. The U gene codes for embryonic hemoglobin; the aD and aA genes code for adult hemoglobin.

Erythroblasts first 24 hours

Erythroblasts 14 days sensitive regions now lie near the genes that produce the adult hemoglobins ( FIGURE 11.9d). DNA from brain cells, which produce no hemoglobin, remains insensitive to DNase digestion throughout development ( FIGURE 11.9e). In summary, when genes become transcriptionally active, they also become sensitive to DNase I, indicating that the chromatin structure is more exposed during transcription.

What is the nature of the change in chromatin structure that produces chromosome puffs and DNase I sensitivity? In both cases, the chromatin relaxes; presumably the histones loosen their grip on the DNA. One process that appears to be implicated in changing chromatin structure is acetylation, a reaction that adds chemical groups called acetyls to the histone proteins. Enzymes called acetyltrans-ferases attach acetyl groups to lysine amino acids at one end (called a tail) of the histone protein. This modification reduces the positive charges that normally exist on lysine and destabilizes the nucleosome structure, and so the his-tones hold the DNA less tightly. Proteins taking part in transcription can then bind more easily to the DNA and carry out transcription.




Images of polytene


Q A chromosome break produces two types of fragments, those with a centromere and those without.

Centromere Structure

The centromere is a constricted region of the chromosome where spindle fibers attach and is essential for proper movement of the chromosome in mitosis and meiosis (Chapter 2). The essential role of the centromere in chromosome movement was recognized by early geneticists, who observed what happens when a chromosome breaks in two. A chromosome break produces two fragments, one with a centromere and one without ( FIGURE 11.10a). In mitosis, the chromosome fragment containing the centromere attaches to spindle fibers and moves to the spindle pole, whereas the fragment lacking a centromere never connects to a spindle fiber and is usually lost because it fails to move into the nucleus of a daughter cell ( FIGURE 11.10b).

Although the centromere's role in chromosome movement has been recognized for some time, its molecular nature has only recently been revealed. The first centromeres to be isolated and studied at the molecular level came from yeast, which have small, linear chromosomes. When molecular biologists attached sequences from yeast centromeres to plasmids (small circular DNA molecules that don't have centromeres), the plasmids behaved in mitosis as if they were eukaryotic chromosomes. This finding indicated that the sequences from yeast, called centromeric sequences ( FIGURE 11.11), contain a functional centromere that allows segregation to take place. Centromeric sequences are the binding sites for proteins that function as the kinetochore, a complex that assembles on the centromere and to which the spindle fibers attach.


Anaphase of mitosis

Telophase of mitosis

In mitosis, each fragment with a centromere attaches to a spindle fiber and moves to the spindle pole,.

In mitosis, each fragment with a centromere attaches to a spindle fiber and moves to the spindle pole,.

Telophase of mitosis

Q ... but fragments without a centromere do not attach to a spindle fiber and are usually lost from the nucleus.

^ After cytokinesis ^

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